Metal spraying



G. J. GIBSON METAL SPRAYING Sept. 5, 1961 2 Sheets-Sheet 1 Filed Sept.ll, 1958 Kim INVENTOR GLENN J. GIBSON Y HM ATTORNEY a AGENT G. J. GIBSONMETAL SPRAYING Sept. 5, 1961 2 Sheets-Sheet 2 Filed Sept. 11, 1958 F IG.

INVENTOR GLENN J. GIBSON BY H. HM maflm M 5w ATTORNEY a AGENT UnitedStates Patent 2,998,922 METAL SPRAYING Glenn J. Gibson, BerkeleyHeights, N.J., assignor to Air Reduction Company, Incorporated, NewYork, N.Y., a corporation of New York Filed Sept. 11, 1958, Ser. No.761,278 3 Claims. (Cl. 239-13) This invention relates to metal sprayingor metallizing and this application forms a continuation-in-part of myco-pending application Serial No. 533,934, filed September 13, 1955, forMethod and Apparatus for Metal Spraying, abandoned. Metal spraying, asthe term implies, is the process by which molten metal is projected in astream or spray which may be directed against a surface to which themolten metal adheres and freezes to form a coating thereon. It isfrequently called metallizing and includes the surfacing of bothmetallic and non-metallic objects.

Metal spraying is generally used where it is desired to have a productwith a metallic surface having different properties than the basematerial. For example, it is frequently advantageous to apply a thincoating of stainless steel or the like to the interior surface of avessel that is to be used to hold some chemically-active fluid. Asanother example, a wear-resistant metal may be applied to a bearingsurface. Similarly, it is sometimes desirable to coat a plastic or othernon-metallic surface with an adhering thin film of metal.

Metal spraying has been accomplished in the past by melting metal inpowder or wire form in a gun, usually by heat derived from a gas flame,and subjecting the molten metal to a blast of gas, usually air, whichatomizes the molten metal and propels it out of the gun as a spray atrelatively high velocity. In such a process the metal is chilled by thepropelling gas blast which effects its ability to make a fusion bondwith the surface against which it is projected, and when an active gasis used as the blast gas, the metal is subject to chemical reaction withthe gas. Even if a relatively inert gas is used as the blast gas, theturbulence of the required high velocity gas blast entrains air andoxidation of the metal is inevitable. This, of course, limits theusefulness of the process.

.An object of the present invention is to provide a method of projectinga superheated arc eflluent into free space.

Another object of the present invention is to provide a novel method ofmetal spraying in which an electric are, employed for melting a supplywire, is operated under conditions which produce by are action alone afree stream of molten metal particles which are projected axially fromthe end of the Wire and from the path of current flow through the arc atvelocities sufiicient to pass a substantial distance from the end of thewire without noticeable gravitational effects.

A further object of this invention is to provide a novel method ofdepositing molten metal in a molten state on a work surface foradherence thereto in the formation of a coating thereon.

These and other objects and advantages of the invention will be pointedout or will become apparent from the following detailed description andthe accompanying drawings.

In the metal spraying method of the present invention, the operatingcharacteristics of a high current density self regulating arc in aninert shielding gas are employed as the sole means of forming andprojecting a stream of molten metal droplets from the end of a consumingwire electrode instead of using a blast of gas for breaking up largeglobules of molten metal formed on the end of a consuming wire electrodeand propelling the resultant spray from the end thereof. Such highcurrent density self regulating arcs have heretofore been employed ininert gas shielded arc welding as disclosed and claimed in my-U.S.Letters Patent 2,504,868, Albert Muller, Glenn I. Gibson and Nelson E.Anderson, patented April 18, 1950, for Electric Arc Welding. In themethod of welding disclosed in this patent the strength of the weldingcurrent, or current density, is such that a spray of fine discrete metaldroplets is projected axially from the end of a consumable electrodewith a force sufficient to overcome the influence of gravity.

According to the present invention, a high current density electric arcis formed between the end of a consuming wire electrode fed toward thearc and a counter electrode positioned off center with respect to theaxis of the consuming wire electrode. The are is maintained in anatmosphere comprising essentially monatomic inert gas. As the end of theconsuming wire electrode melts the molten metal is projectedsubstantially axially with considerable force and velocity by arc actionalone from the end of the electrode and past the counter electrode in astream of fine discrete droplets. This stream may be directed against asurface to which the metal droplets adhere and freeze to form thecoating. Since the coated surface is not involved in the electricalcircuit, it need not be an electrical conductor.

In order to more fully understand the novel method and apparatusreference should be made to the accompanying drawings.

FIGURE 1 illustrates generally a complete apparatus for the practice ofthe present invention.

FIGURE 2 illustrates in more detail the metal spray gun forming part ofthe apparatus of FIGURE 1.

FIGURE 3 illustrates apparatus that may also be used in practicing themethod of this invention.

FIGURE 4 is an enlarged view of a portion of the apparatus of FIGURE 3,showing in more detail the relation of the consuming wire electrode tothe non-consuming counter electrode.

Referring to FIGURE 1 of the drawings, the metal spray gun is designatedgenerally by the reference numeral 11. Wire W is fed to the gun 11 froma reel 12 by a feed motor 13 and feed rolls 14 driven thereby. The wirereel and the wire feeding equipment are supported, along with otherassociated apparatus, in a wire feed carriage frame 16. The wire W,withdrawn from the reel by the feed rolls, is pushed through a flexiblecasing 17 to a spray gun. Arc current is supplied to the spray gun 11from a power source such as a conventional welding power source 18. Inthe illustrated apparatus one lead 24 from the power source 18 connectswith a power switch or contactor 26 which, in turn, is connected to thespray gun by conductor 27. The other lead 28 from .theswelding machineisconn ected directly to the spray gun 11. Shielding gas may be providedto the spray gun from a conventional compressed gas cylinder 19 throughthe necessary pressure reducing valve 21 and gas conduit 22. Coolingwater is supplied to the apparatus from a pipe line source 30 throughconduit 29 for circulation through the spray gun and subsequentdischarge through conduit 31.

Details of the spray gun 11 are more clearly shown in FIGURE 2. The gunconsists generally of a pistol grip handle 32, an outer cylindricalbarrel 33, and an inner barrel 35. The inner barrel 35 terminates in acontact tube 34. The annular passage 36 between the inner-and-outerbarrel serves as'a gas passage for they flow therethrough of arcshielding gas. The contact tube 34 is in electrical circuit withconductor '27. The annular gas passage 36 communicates with gas conduit22. Barrel 33 is formed of non-conducting material such as a moldablesynthetic resin. An adapter 37 is screwed into the end of barrel 33 andprovides the necessary means to attach a counter electrode and gasnozzle assembly 38. This unit consists of a cylindrical nozzle 39 ofhigh conductivity copper which is flanged at one end to be attachedtothe adapter 37 by a connector nut 41. The counter electrode itselfconsists of a heavy copper ring 42 which is electrically andmechanically bonded to the inside of the nozzle 39 as by welding or anequivalent metal bonding process. A jacket 43 is soldered tothe outsideof nozzle 39.0f "form a cooling water passage through which coolingwater maybe circulated in direct heat exchange relation with the nozzle39. Cooling water is supplied to this jacket from conduit 29 throughacopper tube 44 and it discharges from the jacket through a similarcopper tube-45 into the discharge conduit'3l. These copper tubes 44, 45also serve to connect the counter electrode 42 through the cooling waterjacket 43 and the nozzle 39 to the conductor 28 connected to oneterminal of the power supply 18.

In operation the above described apparatus functions in the followingmanner. An arc is established between the end of the wire electrode Wand the counter electrode 42 with current supplied from the power source18. The are may conveniently be started by bridging the gap between thewire electrode and the counter electrode with a carbon rod or the like,or it may 'be .started by deflecting the wire to touchthe counterelectrode momentarily. Alternatively, the arc may be started by a highvoltage, high frequency discharge if appropriate conventional apparatus(not shown) is employed. Wire is withdrawn from reel 12 by the motordriven feed rolls .14 andpushed through thecasing 17 and the spray gun11 to emerge from the contact tube 34 at a rate to maintain the are asmetal is meltedfrom :the end of the wire. Arc current is transferredfrom the contact tube 34 to the wire Was the wire passes through thecontact tube in contact therewith. Shielding :gas, preferably comprisingessentially inert monatomic gas such as argon, is supplied to theannular gaspassage36inthe gun from cylinder 19 through conduit 22. Thisgas .passes along the inner barrel and the contact tube to theprojecting end of the electrode andapasses out of the gun through nozzle39. At the high current densities .employed, the high temperatureproducts formed by .the inert gas shielded are established between theelectrode rod or Wire 'and the'boundary surfaces of the elongated nozzleof gun 11 are projected from the nozzle into free space as a superheatedefiiuent which extends a substantial distance beyond the end of thenozzle. The power supply 18 is preferably a direct current power supplywith its positive terminal in electrical contact with the electrode wireW via the contact tube 34 in'the spray gun and its negative terminal inelectrical contact with the counter electrode 42in the spray gun nozzle.It has been found that when the arc current is maintained at asufficiently high level, Whichin all instances results-in burn-off ofthe electrode wire at=a rate of :at least 100 inches .per minute moltendroplets are projected axially from the endof thewire W with substantialvelocity. For example, using 20 cubic feet per hour of argon as theshielding gas through a inch diameter nozzle in the apparatus shown,with a .035 inch diameter stainless steel wire W and a welding currentof the order of 200 amperes, molten droplets are projected horizontallyfrom the spray gun into the air six to eight feet from the nozzle of thegun before falling to the floor when the gun is held at chest height bythe operator. The force propelling these molten metal droplets is due toare action alone since the flow of shielding gas is only about 2.6 feetper second which is obviously incapable of acting as a blast gas such asheretofore been employed. Since an objective of this invention is toprovide a metal spraying method and apparatus, normal operation requiresthe gun to be held within a few inches of the surface to be sprayed.Under the recited conditions the projection velocity of the droplets isalways sufiicient to cause the droplets to traverse the distance to thework surface without any noticeable gravitational effect. The projectedmolten metal droplets are superheated and form a true fusion bond with ametal work surface, although little or no dilution of the depositedmetal occurs. This spray gun may be used in all positions equally well.When the gun is held near the surface to be coated, the inert shieldinggas emerging from the nozzle forms an envelope protecting the moltenmetal particles as they are projected by are action across the spacefrom the gun to the workpiece and it also serves to prevent oxidation ofthe metal after it strikes the workpiece and as it freezes thereon. Themanipulation of the gun depends entirely on the job being perf ormed,but in general it may be used in much the same manner as any other spraygun, i.e. by making a series of traverses across the surface to besprayed to thereby deposit a layer of substantially uniform thickness.While the specific examples described in this specification relate tothe use of stainless steel electrode wire, it is to be understood thatother metals and alloys can and have been used with equal success.

An important factor in this novel metal spraying method and apparatus isthe combination of are conditions that produce projected metal transferfrom the end of the wire electrode and the proper orientation of thewire electrode and the counter electrode such that a stable arc ismaintained while permitting unimpeded passage of the molten metaldroplets in an axial direction from-the end of the wire electrode.

The construction and mode of operation described above accomplishes theresult in an entirely satisfactory manner. However, many modificationswill become apparent to one skilled in the art. For example, undernormal circumstances the arc current distributes itself substantiallyuniformly over the entire circumference of the water-cooled coppercounter electrode. It has been found, however, that due to lack ofsymmetry of construction or for some other reason the arc may tend toconcentrate on one spot on the ring. This is undesirable because of theexcess local heating that results. This may be overcome by providing arotating magnetic field to cause the arc to rotate rapidly around thering. It is also contemplated to reduce the heat produced at the counterelectrode by adding an emissive agent to the arc. In the preferred formof the invention th counter electrode is the .cathode and anymodification to improve its electron emission properties is desirable.

The composition of the shielding gas is important in that it determinesto a large extent the nature of the metal transfer from the end of thewire as well aspro- "viding a medium to exclude air from the region ofthe arc. Inert monatornic gas such as argonhas been found particularlywell suited for use in the invention, although other inert gases as wellas mixtures of inert gas and small amounts of active gas iscontemplated;

Other variations in the invention include the use of an unbalancedcounter electrode. Instead of having the are formed to a ring throughthe center-of which the molten metal projects, the counter electrode maybe a rod offset to one side of the wire axis. Apparatus of this type isshown in FIGURES 3 and 4.

In the particular application of the invention illustrated in FIGURES 3and 4 a round workpiece 51 is being surfaced by rotating it under ametal spraying head made and operated according to the presentinvention. The metal spraying head may be fixed with respect to therotation of the workpiece and may be provided with a longitudinal feedto move it axially along the work as the deposit is formed duringsuccessive revolutions of the work. This head comprises essentially awire feeder and barrel for the consuming electrode and a holder for anon-consuming counter electrode. The wire feeder for the consumingelectrode W may be of conventional design for inert gas shieldedconsuming electrode metal arc welding as exemplified by the apparatus ofU.S. Letters Patent 2,504,868 referred to above. It consists essentiallyof a wire reel 52 containing a supply of electrode wire W, a wirefeeding head 53, including wire feed rolls 54 driven by an electricmotor 55, and a barrel 56 through which the wire is fed and whichcontains a contact shoe fromwhich the electrode W collects weldingcurrent. The barrel 56 also provides an annular passage for the flow ofshielding gas to a nozzle 57 which delivers the gas as a flowing streamsurrounding the arc end of the electrode wire. Shielding gas may beprovided to the barrel through an appropriate connection as shown in thedrawing. Arc current is provided from a power source 60. In thismodification of the invention the counter electrode con sists of atungsten electrode 61 or alternatively some other non-consumingelectrode such as carbon, zirconium, or the like. This electrode issupported by an electrode holder 62 which may be of conventional designfor inert gas shielded tungsten electrode arc welding as shown, forexample, in U.S. Letters Patent 2,512,705, Nelson E. Anderson and GeorgeR. Turbett, granted June 27, 1950, for Fluid-Cooled Gas-blanketed ArcWelding Torch. It con sists essentially of means to conduct arc currentto the counter electrode and means to deliver shielding gas around theelectrode to exclude air therefrom. The are current terminal of thecounter electrode holder and the arc current teminal on the consumingelectrode barrel are connected to the respective terminals of the powersource 60 which in the preferred from of the invention is a directcurrent source and which preferably has its negative terminal connectedto the non-consuming counter electrode and its positive terminalconnected to the consuming wire electrode. In general, the orientationof the two electrodes is such that the end of the tungsten is olfsetfrom the axis of the consuming wire W. As used in the presentspecification and claims, the term axis, as applied to the wire, meansthe longitudinal centerline of the wire and its indefinite projectionbeyond the end of the wire.

It has been found from experiment that a number of factors influence thetype and direction of the metal spray produced by this apparatus. Thesefactors include the angle between the two electrodes, the distance fromthe end of the counter electrode to the axis of the consuming electrode,the strength of the arc current, particularly as it relates to thediameter of the wire electrode, and the wire feed speed. FIGURE 4 showsthe relationship of the two electrodes for optimum results for mostmetal spraying applications. The angle a between the axes of theelectrodes should be approximately 90, the end of the tungsten electrodeshould be offset from the axis of the consuming wire electrode by aboutinch, and the current density and wire feed speed should be adjusted sothe consuming wire electrode assumes a tapered form at the end whichprojects slightly beyond the end of the nonconsuming electrode. Thiscondition is attained, for example, using 385 amperes with a li inchdiameter stainless steel wire, with the wire the anode, an arc voltageof 37 volts, and a wire feed speed of 400 inches per minute. Under theseconditions, a fine spray of discrete droplets is projected substantiallyaxially from the end of the wire electrode W at a relatively highvelocity, and well within the shielding envelope formed by theconverging shielding gas streams. By placing the electrodes so that theangle at angle between them is substantially less than or substantiallygreater than the projection ve locity is decreased and the molten metalstream becomes more dispersed. The angle on is preferably always morethan 60 and less than Obviously, there are many alternatives to themodification of the invention described immediately above. For instance,it is advantageous to use as a counter electrode a material that willoperate as a cathode with a minimum temperature rise, such as thoriatedor zirconiated tungsten. Also, instead of having separate consumingelectrode and counter electrode holders, a single head can be provided'which will serve both functions, and instead of having separate gasshielding apparatus for each electrode both may be shielded by a singlenozzle of proper design.

While only certain specific embodiments of the invention have been shownand described in connection with methods and apparatus for metalspraying, it is to be understood that the invention is not limited tothe particular forms shown and described, but may be used in other wayswithout departure from its spirit as defined by the following claims.

Furthermore, the invention is not limited to forming metal coatings onworkpieces. It may be used for other purposes such as forming metalgranules which may be used for example in shot blasting workpieces toproduce suitable surface conditions thereon. The molten metal dropletsformed by the process disclosed may be solidified to form the desiredmetal granules by air cooling, cooling in a shielded atmosphere, or bybeing directed into a cooling liquid such as water.

I claim:

1. The method of projecting into free space the fluid products of anelectric arc struck in an inert gaseous medium between a bare wireconsuming electrode connected to a source of direct current and acounter electrode also connected to said source and offset with respectto the longitudinal axis of said wire electrode, characterized byfeeding said wire electrode toward said are at a rate of at least 100inches per minute, and simultaneously supplying current from said sourceto said electrodes with said wire electrode at positive polarity andsaid current at a strength suffioient to both consume said wireelectrode at said rate and also project the fluid arc products axiallyfrom said wire electrode past the said counter electrode.

2. The method which comprises flowing an inert gas through an annularpassage around an electrical contact element and then over the exposedend portion of a consuming electrode projecting from said contactelement and out through a nozzle having an inner annular metal surfaceportion which constitutes both a counterelectrode and an exit orificethrough which said inert gas flows into the free space beyond saidnozzle, said annular counterelectrode being positioned concentricallywith respect to the axis of said consuming electrode and being spacedlaterally therefrom to form an arc gap extending generally transverselyacross the flow path of the said inert gas flowing through said exitorifice, and establishing a direct current arc between the end of saidconsuming electrode and said annular counterelectrode at a current levelwhich is sufficient to consume the said electrode at a rate of at least100 inches per minute while projecting the hot fluid products of saidare axially of said electrode out through the said exit orifice into thefree space therebeyond while maintaining the said arc rooted at one endon the said electrode and at the other end on said counterelectrode.

3. The method which comprises positioning the exposed end of a firstconsuming electrode in coaxial relationship with the inner surface of anannular non-con- 7 t s a s suming counterelectrode to form an arc gap,th ereberespect to said first electrode and out through the oritween,flowing an inert gas over the surface of the exfice formed by the innersurface of said counterelectrode posed end ofi-said "first electrode andthe inner surface into free space therebeyond. of said 'counterelectrodeand through said are gap to shield the said electrode surfaces and forman inert gas 5 References Cited in the file ofthispatent atmosphere insaid-arc gap, and establishing-an arc across UNITED STATES PATENTSsaidarc gap at a current 'level of the order of 200 amy I peres or moreto consume said first electrodewhile mainiz fi g taining the said arerooted at one'end on said first elecer e 1950 frolic-and at'the otherend on saidcounterelectrode while 10 4.1 4 3 33 X 1952 projecting-thehigh temperature are products axially with

